1. Field of the Invention
The present invention relates generally to a multi-carrier transmission system a transmission unit and a transmission method. More particularly, the invention relates to a multi-carrier transmission system, a transmission unit and a transmission method known as a Discrete Multi-Tone (DMT) modulation system.
2. Description of the Related Art
As an example of the conventional DMT type multi-carrier transmission system of this kind, there is a technology disclosed in U.S. Pat. No. 5,479,447. Such multi-carrier transmission system measures a signal to noise ratio (SNR) of respective carriers for bit distribution of a plurality of carriers, and for deriving bit distribution according to the measured SNR.
A problem in such prior art resides in small transmission amount. The reason is that when a periodically varying noise is generated and if communication is to be established at a certain error rate, bit distribution and power distribution is performed with reference to an average value of SNR of each carrier. Thus, bit distribution and power distribution are limited to only one kind of average value of SNR to inherently make the data transmission amount small.
Therefore, the present invention has been worked out for solving the drawback in the prior art. An object of the present invention to provide a multi-carrier transmission system, a transmission unit and a transmission method, which can efficiently perform data transmission in a condition where a noise is generated periodically.
According to one aspect of the present invention, a multi-carrier transmission system is characterized by performing data transmission by deriving a bit distribution of each carrier of multi-carrier depending upon a periodically varying noise period. Then, the bit distribution of each of the carrier is derived by using a clock synchronized with the periodically varying noise and depending upon a period of the clock. A transmission power distribution of each of the carrier is derived together with the bit distribution of each of the carrier.
Another multi-carrier transmission system according to the present invention, comprises:
a first unit performing quadrature amplitude modulation using each of the carrier and multiplexing modulated carrier using inverse Fourier transformation; and
a second unit demodulating each of the carrier from the signal multiplexed in the first unit using Fourier transformation.
On the other hand, the first unit comprises a circuit for transmitting a tone synchronized with the clock with varying level and a circuit for transmitting a pseudo random signal having all components of respective carriers, and
the second unit comprises a circuit for detecting a period of noise on the basis of the level of the tone from the first unit, a circuit for deriving a signal to noise ratio of the pseudo random signal per detected period and a circuit for deriving one of a transmission power distribution and bit distribution, and bit distribution of the carrier on the basis of the signal to noise ratio. Then, the second circuit comprises a circuit for transmitting any one of derived transmission power distribution and the bit distribution, and the bit distribution of each of the carrier to the first unit, and the first unit comprises a circuit for performing data transmission to the second unit according to one of the transmission power distribution and bit distribution, and the bit distribution per the noise period from the second unit. The second unit comprises a circuit for transmitting a pseudo random signal having all components of each of the carriers, and the first unit comprises a circuit for deriving a signal to noise ratio of the pseudo random signal from the first unit per each noise period, and a circuit for deriving one of a transmission power distribution and bit distribution, and bit distribution of the carrier on the basis of the signal to noise ratio. The first unit comprises a circuit for transmitting the transmission power distribution and the bit distribution of each of the carriers derived to the second unit, the second unit comprises a circuit for performing data transmission to the first unit according to one of the transmission power distribution and bit distribution, and the bit distribution per the noise period from the first unit. The first unit initiates transmission operation in response to a transmission start command for own unit from the second unit. A data transmission through a digital subscriber line is performed between the first unit and the second unit. The bit distribution is effected to distribute greater number of bits when the noise periodically varying between large and small is small than that when the noise is large. The bit distribution is effected to distribute greater number of bits when the noise periodically varying is far end crosstalk than that when the noise is near end crosstalk.
According to the second aspect of the present invention, a multi-carrier transmission unit performing transmission by multi-carrier, characterized by performing data transmission by deriving a bit distribution of each carrier of multi-carrier depending upon a period of periodically varying noise. The bit distribution of each of the carrier is derived by using a clock synchronized with the periodically varying noise and depending upon a period of the clock. A transmission power distribution of each of the carrier is derived together with the bit distribution of each of the carrier. The multi-carrier transmission unit performs quadrature amplitude modulation using the each carrier and multiplies the modulated carrier using inverse Fourier transformation.
On the other hand, the multi-carrier transmission unit further comprises a circuit for transmitting a tone synchronized with the clock with varying level and a circuit for transmitting a pseudo random signal having all components of respective carriers.
The multi-carrier transmission unit further comprises a circuit for detecting a period of noise on the basis of the level of the tone from another unit, a circuit for deriving a signal to noise ratio of the pseudo random signal per detected period and a circuit for deriving one of a transmission power distribution and bit distribution, and bit distribution of the carrier on the basis of the signal to noise ratio. The multi-carrier transmission unit further comprises a circuit for transmitting any one of derived transmission power distribution and the bit distribution, and the bit distribution of each of the carrier to the another unit. The multi-carrier transmission unit further comprises a circuit for deriving a signal to noise ratio of the pseudo random signal from the another unit per each noise period, and a circuit for deriving one of a transmission power distribution and bit distribution, and bit distribution of the carrier on the basis of the signal to noise ratio. The multi-carrier transmission unit further comprises a circuit for transmitting one of the transmission power distribution and the bit distribution, and the bit distribution of each of the carriers derived, to the another unit.
The multi-carrier transmission unit initiates transmission operation in response to a transmission start command for own unit from the another unit. A data transmission through a digital subscriber line is performed between own unit and the another unit. The bit distribution is effected to distribute greater number of bits when the noise periodically varying between large and small is small than that when the noise is large. The bit distribution is effected to distribute greater number of bits when the noise periodically varying is far end crosstalk than that when the noise is near end crosstalk.
According to the third aspect of the present invention, a multi-carrier transmission method characterized by performing data transmission by deriving a bit distribution of each carrier of multi-carrier depending upon a periodically varying noise period. The bit distribution of each of the carrier is derived by using a clock synchronized with the periodically varying noise and depending upon a period of the clock. A transmission power distribution of each of the carrier is derived together with the bit distribution of each of the carrier.
In the method set forth above, quadrature amplitude modulation is performed by using each of the carrier and the modulated carrier is multiplexed using inverse Fourier transformation in a first unit, and each of the carrier is demodulated from the signal multiplexed in the first unit using Fourier transformation in a second unit. In the first unit, a tone is transmitted synchronized with the clock with varying level and a pseudo random signal having all components of respective carriers is transmitted, and in the second unit, a period of noise is detected on the basis of the level of the tone from the first unit, a signal to noise ratio of the pseudo random signal per detected period is derived and one of a transmission power distribution and bit distribution, and bit distribution of the carrier is derived on the basis of the signal to noise ratio.
Also, in the second circuit, any one of derived transmission power distribution and the bit distribution, and the bit distribution of each of the carrier is transmitted to the first unit, and in the first unit, data transmission to the second unit is performed according to one of the transmission power distribution and bit distribution, and the bit distribution per the noise period from the second unit.
Furthermore, in the second unit, a pseudo random signal having all components of each of the carriers is transmitted, and in the first unit, a signal to noise ratio of the pseudo random signal from the first unit per each noise period is derived, and one of a transmission power distribution and bit distribution, and bit distribution of the carrier on the basis of the signal to noise ratio is derived. In the first unit, the transmission power distribution and the bit distribution of each of the carriers derived to the second unit is transmitted, and in the second unit, data transmission to the first unit is performed according to one of the transmission power distribution and bit distribution, and the bit distribution per the noise period from the first unit.
The transmission operation is initiated in response to a transmission start command for own unit from the second unit. A data transmission through a digital subscriber line is performed between the first unit and the second unit. The bit distribution is effected to distribute greater number of bits when the noise periodically varying between large and small is small than that when the noise is large. The bit distribution is effected to distribute greater number of bits when the noise periodically varying is far end crosstalk than that when the noise is near end crosstalk.
In the operation of the present invention, a central office and a terminal forming the transmission system according to the invention, both have a transceiver function for performing mutual transmission. In an initial operation started by connection of the terminal to a transmission line, the power distribution and bit distribution of each carrier is calculated depending upon the noise varying a given period (assumed to be known). Therefore, the period of the noise has to be noticed from the center office side as higher level office to the terminal as lower level office. Accordingly, the level of the tone is transmitted from the center office to the terminal with varying the level by the clock synchronized with noise period.
In the terminal, the clock is generated in synchronism with noise depending upon the level of tone. The SNR of the pseudo random signal including all carriers transmitted from the center office is measured to calculate the power distribution and bit distribution of each carrier per noise period depending upon SNR. Then, the power distribution and bit distribution of each carrier per noise period are transmitted from the center office to the terminal.
In the center office, according to derived power distribution and bit distribution of each carrier per noise period, transmission (down) to the terminal is performed. Concerning up transmission, similar procedure is performed with reverse functions of the center office and the terminal.
Thus, by performing the power distribution and the bit distribution according to the period of noise, bit distribution adapted to the noise becomes possible to realize efficient transmission.
On the other hand, when power distribution and bit distribution of each carrier by measuring the SNR, crosstalk noise has to be considered. In general, crosstalk includes far end crosstalk and near end crosstalk. These will be discussed with reference to FIG. 6. In FIG. 6, the crosstalk taking the signal flowing through the signal line 61 in the same direction as the signal line 60 as an object of measurement, as crosstalk source, is far end cross talk FEXT. Since the signal to be transmitted is attenuated according to transmission distance, the amount of far end crosstalk FEXT is also attenuated depending upon transmission distance. Accordingly, when far end crosstalk FEXT is caused, the SNR should show high value irrespective of transmission distance.
On the other hand, the crosstalk taking the signal line 62, in which the signal flows in the opposite direction to the signal line 60 to be the object of measurement, as crosstalk source, is near end crosstalk NEXT. In contrast to attenuation of the signal to be transmitted according to increasing of the transmission distance, the crosstalk amount of the near end crosstalk NEXT is large in the transmission destination to show low SNR value.
Accordingly, when far end crosstalk with high SNR is caused, power distribution and bit distribution of each carrier is increased, and when near end crosstalk with low SNR is caused, power distribution and bit distribution of each carrier is made small.